The growth of the sophisticated information-oriented society in recent years has been accompanied by rapid advances in the manufacture of finer and more highly integrated elements. Lenses of higher numerical apertures are being used in order to deal with the manufacture of finer elements in demagnifying projection exposure systems used in the manufacture of semiconductor devices. Though resolution rises owing to use of higher numerical apertures, however, effective focal depth decreases with an increase in numerical aperture and higher integration. In order to assure a sufficiently practical focal depth while maintaining resolving power, it is necessary to mitigate curvature of the image surface in the projection optical system, improve upon uniformity of substrate thickness and raise the precision of the chuck plane.
A method that minimizes the rate of contact between the underside of a wafer and the vacuum-retention side of a chuck is adopted conventionally as effective means for suppressing element defects ascribable to foreign matter. In particular, a pin-contact-type chuck that comes into point contact with the underside of the wafer is becoming increasingly in vogue.
The structure of an ordinary pin chuck is illustrated in FIG. 2. The ordinary pin chuck has an annular seal 14 on its outer circumference and a multiplicity of 0.2-mm circular or square pin-shaped contact portions (referred to also as “pin-shaped protrusions” below) 12 dispersed inwardly of the seal at a pin pitch of 2 mm. Further, in general, the circumferential seal 14 and a seal portion 13 of a hole 11 for a substrate lifting pin each define a continuous, embankment-like peripheral wall. When this pin chuck is used, three problems ascribable to vacuum-induced deformation arise.
The first problem is wafer flexure that occurs between the pin-shaped protrusions. In a case wherein a wafer is held by suction through pin-like contact, flexure occurs owing to the action of an external deforming force caused by the suction force of the vacuum with regard to the pin-to-pin intervals. For example, if pin spacing is 2 mm, flexure on the order of 5 nm occurs. This amount of flexure is not negligible in view of the specifications that will be demanded of exposure systems in the future owing to the use of higher numerical apertures and shorter wavelengths.
The second problem is doming deformation ascribable to wafer flexure that occurs at the portion of the lifting-pin hole 11. As a result of such deformation, a large dome in excess of 100 nm is caused by wafer deformation between the peripheral wall portion of the seal for the lifting-pin hole and the adjoining pins, flexure of the pins themselves and digging of the pins into the wafer.
The third problem is lift-up, which occurs for reasons similar to those of the problems above, between the seal wall (14) at the outer circumference of the chuck and the adjacent pins. The circumferential portion undergoes a large amount of lift-up produced because wafer deformation acts on the free end. There are cases where this lift-up exceeds 300 nm.
In order to deal with the first problem, it has been attempted to array the chuck protrusions in the form of a grid or concentric circles and reduce the spacing between the pins. However, if the pins are brought closer together by reducing pin spacing in order to reduce flexure between the pins, the amount of flexure decreases but the rate of contact with the underside of the wafer increases, thereby elevating the probability that foreign matter will intrude.
This type of deformation between pins does not exhibit positional correlation with respect to the exposure viewing angle of the exposure apparatus. As a consequence, pin contact position differs for every exposure viewing angle and deformed shape ascribable to deformation between the pins is not reproduced from one exposure shot to the next. As a result, the amount of variation in focus increases for every exposure viewing angle. Usually, in an exposure apparatus that performs exposure using a square or rectangular angle of view, the above-mentioned variation is more pronounced with the concentric circular array than with the grid-type array.
With regard to the second and third problems, the prior art is such that a decline in planarity at the time of vacuum-induced suction becomes conspicuous at the periphery of the hole for the substrate lifting pin near the center of the chuck. For this reason, it has been proposed to provide the seal portion with a difference in level (e.g., see the specification of Japanese Patent Application Laid-Open No. 10-233433), and a chuck in which all contacting portions employ point contact also has been proposed (e.g., see the specification of Japanese Patent Application Laid-Open No. 8-195428). These chucks are such that the peripheral wall portion of the seal for assuring vacuum is formed to be one level lower than the tops of the pins, and a plurality of protuberances are provided on the peripheral wall portion. In each of these examples of the prior art, however, leakage at the peripheral wall portion is a problem and various difficulties arise, such as a decline in vacuum pressure and loss of a plane correcting force at the rim of a wafer that exhibits a large amount of curvature. In addition, in order to lower the peripheral wall portion with stabilized dimensions, highly precise partial machining is required. This results in higher manufacturing cost.
Accordingly, in view of the state of the prior art set forth above, there is a need for the provision of a chuck in which excellent planarity is obtained between pins, in the vicinity of the lifting pin and at the rim of the chuck.